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<h1 id="mir-debugging"><a class="header" href="#mir-debugging">MIR Debugging</a></h1>
<p>The <code>-Z dump-mir</code> flag can be used to dump a text representation of the MIR.
The following optional flags, used in combination with <code>-Z dump-mir</code>, enable
additional output formats, including:</p>
<ul>
<li><code>-Z dump-mir-graphviz</code> - dumps a <code>.dot</code> file that represents MIR as a
control-flow graph</li>
<li><code>-Z dump-mir-dataflow</code> - dumps a <code>.dot</code> file showing the <a href="./dataflow.html#graphviz-diagrams">dataflow state</a> at
each point in the control-flow graph</li>
</ul>
<p><code>-Z dump-mir=F</code> is a handy compiler option that will let you view the MIR for
each function at each stage of compilation. <code>-Z dump-mir</code> takes a <strong>filter</strong> <code>F</code>
which allows you to control which functions and which passes you are
interested in. For example:</p>
<pre><code class="language-bash">&gt; rustc -Z dump-mir=foo ...
</code></pre>
<p>This will dump the MIR for any function whose name contains <code>foo</code>; it
will dump the MIR both before and after every pass. Those files will
be created in the <code>mir_dump</code> directory. There will likely be quite a
lot of them!</p>
<pre><code class="language-bash">&gt; cat &gt; foo.rs
fn main() {
println!("Hello, world!");
}
^D
&gt; rustc -Z dump-mir=main foo.rs
&gt; ls mir_dump/* | wc -l
161
</code></pre>
<p>The files have names like <code>rustc.main.000-000.CleanEndRegions.after.mir</code>. These
names have a number of parts:</p>
<pre><code class="language-text">rustc.main.000-000.CleanEndRegions.after.mir
---- --- --- --------------- ----- either before or after
| | | name of the pass
| | index of dump within the pass (usually 0, but some passes dump intermediate states)
| index of the pass
def-path to the function etc being dumped
</code></pre>
<p>You can also make more selective filters. For example, <code>main &amp; CleanEndRegions</code>
will select for things that reference <em>both</em> <code>main</code> and the pass
<code>CleanEndRegions</code>:</p>
<pre><code class="language-bash">&gt; rustc -Z dump-mir='main &amp; CleanEndRegions' foo.rs
&gt; ls mir_dump
rustc.main.000-000.CleanEndRegions.after.mir rustc.main.000-000.CleanEndRegions.before.mir
</code></pre>
<!--- TODO: Change NoLandingPads. [#1232](https://github.com/rust-lang/rustc-dev-guide/issues/1232) -->
<p>Filters can also have <code>|</code> parts to combine multiple sets of
<code>&amp;</code>-filters. For example <code>main &amp; CleanEndRegions | main &amp; NoLandingPads</code> will select <em>either</em> <code>main</code> and <code>CleanEndRegions</code> <em>or</em>
<code>main</code> and <code>NoLandingPads</code>:</p>
<pre><code class="language-bash">&gt; rustc -Z dump-mir='main &amp; CleanEndRegions | main &amp; NoLandingPads' foo.rs
&gt; ls mir_dump
rustc.main-promoted[0].002-000.NoLandingPads.after.mir
rustc.main-promoted[0].002-000.NoLandingPads.before.mir
rustc.main-promoted[0].002-006.NoLandingPads.after.mir
rustc.main-promoted[0].002-006.NoLandingPads.before.mir
rustc.main-promoted[1].002-000.NoLandingPads.after.mir
rustc.main-promoted[1].002-000.NoLandingPads.before.mir
rustc.main-promoted[1].002-006.NoLandingPads.after.mir
rustc.main-promoted[1].002-006.NoLandingPads.before.mir
rustc.main.000-000.CleanEndRegions.after.mir
rustc.main.000-000.CleanEndRegions.before.mir
rustc.main.002-000.NoLandingPads.after.mir
rustc.main.002-000.NoLandingPads.before.mir
rustc.main.002-006.NoLandingPads.after.mir
rustc.main.002-006.NoLandingPads.before.mir
</code></pre>
<p>(Here, the <code>main-promoted[0]</code> files refer to the MIR for "promoted constants"
that appeared within the <code>main</code> function.)</p>
<p>The <code>-Z unpretty=mir-cfg</code> flag can be used to create a graphviz MIR
control-flow diagram for the whole crate:</p>
<p><img src="mir_cfg.svg" alt="A control-flow diagram" /></p>
<p>TODO: anything else?</p>
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